The present invention relates in general to battery charger oil circuitry, and in particular to improved method and circuitry for controlling battery chargers that reduce circuit complexity and prevent undesirable oscillations.
A typical method of charging a battery for a portable consumer application involves providing a constant charging current into the battery until the battery reaches a certain voltage. The voltage is then regulated with a voltage control loop at a constant level causing the current to gradually reduce over time. This is usually accomplished by controlling battery charger circuitry with a combination of a current feedback amplifier and a voltage feedback amplifier as shown in FIG. 1A. To recharge the battery in the shortest possible time, it is common to use the maximum available current for charging. As the battery approaches the point at which a switch to voltage regulation is to occur, however, an unwanted side-effect may occur. The battery typically has a built-in over-voltage protection switch 100. Since the battery has a non-zero impedance, the high charging current makes the terminal voltage appear higher than it would be if the battery were open circuit. This may cause the over-voltage protection switch to open (FIG. 1B). When this happens, the voltage rises rapidly as the large charging current flows into the open circuit. This causes the voltage control loop to take over, greatly reducing the charging current (FIG. 1C). With reduced charge current, the battery terminal voltage reduces, causing the over-voltage protection switch to close (FIG. 1D). By this point, however, there is very little current flowing into the battery and the voltage begins to drop. Because of the dropping voltage level, the current loop takes over (FIG. 1E). Depending on the large-signal bandwidth of the current control loop, the charge current to the battery may greatly overshoot. This is because the current loop amplifier saturates at its rail voltage when it is not in control and requires time both to desaturate and to slew to its appropriate output level. Whether there is overshoot or not, this cycle may repeat continually resulting effectively in an oscillating system.
One solution to this problem has been the use of the so-called soft-start of the system controller. When an over-voltage condition is detected, the soft-start is reset. Thus, when the current loop comes back into control, even though it is demanding a very high current, the soft-start prevents this by limiting the duty cycle; and the duty cycle is allowed to increase only slowly, so that the current increases slowly. This avoids the current overshoot, and can also be made to avoid oscillations.
A drawback of this approach is that it severely limits the actual, large-signal bandwidth at the current control loop. Additionally, since it introduces a non-linear element in a linear system, it may be capable of causing its own oscillations under certain circumstances. Finally, combining two functions into one (soft-start and oscillation suppression) renders the circuitry more complex, may create difficulties in component selection, and can be confusing for a designer.